1. Field of the Invention
The present invention relates to a spherical aberration correction control device and an optical disc apparatus, and in particular to a spherical aberration correction control device for controlling a spherical aberration correction section for correcting a spherical aberration of an optical beam irradiating an optical disc, and an optical disc apparatus including the spherical aberration correction control device.
2. Description of the Related Art
Recently, optical discs such as DVD-ROM discs, DVD-RAM discs, DVD-RW discs, DVD-R discs and DVD+RW discs and DVD+R discs have been developed as high density, large capacity recording mediums.
An optical disc apparatus for recording information on such optical discs or reproducing information recorded on such optical discs generally uses an optical lens (objective lens) having a numerical aperture (NA) of 0.6. In order to realize a higher density and a larger capacity in recording information on or reproducing information from the optical discs, it has been proposed to use an optical lens having a numerical aperture of 0.8 or greater.
When using an optical lens having a numerical aperture of 0.8 or greater in an optical disc apparatus for accessing the data on an optical disc, generation of a spherical aberration cannot be ignored, which is not a problem when an optical disc having a numerical aperture of 0.6 is used. The spherical aberration is caused by a non-uniform thickness of a protective layer which protects an information face of the optical disc.
Currently, in order to eliminate the influence of the spherical aberration, the spherical aberration is corrected, or the correction amount of the spherical aberration is switched. Such correction of the spherical aberration and switching of the correction amount of the spherical aberration are especially effective when the optical disc has a plurality of information faces (see, for example, Japanese Laid-Open Publication No. 2002-157750, paragraphs 0070 through 0101 and FIGS. 1 through 8).
FIG. 33 is a timing diagram illustrating a conventional operation for switching the correction amount of the spherical aberration. Part (a) shows a time-wise change in a spherical aberration correction lens driving signal, and part (b) shows a time-wise change in a correction amount of the spherical aberration in consideration of the thickness of the protective layer. Part (c) shows a time-wise change in a moving speed of a spherical aberration correction lens, and part (d) shows a time-wise change in a moving acceleration of the spherical aberration correction lens.
In order to switch the correction amount of the spherical aberration to a target correction amount, the spherical aberration correction control device is conventionally operated as follows. As shown in part (a) of FIG. 33, the spherical aberration correction lens driving signal is switched in one step. As shown in part (b), the correction amount of the spherical aberration fluctuates with respect to the target correction amount when the spherical aberration correction lens driving signal is switched. In other words, the post-switching correction amount has a fluctuation component with respect to the target correction amount. The fluctuation component of the post-switching correction amount decreases in accordance with time, and as a result, the post-switching correction amount is converged to the target correction amount in a prescribed time period.
According to the conventional technique of correcting the spherical aberration, the correction amount of the spherical aberration is switched to a target correction amount as described above. The post-switching correction amount has a fluctuation component with respect to the target correction amount, and the fluctuation amount requires a prescribed time period to become zero. Therefore, it is time-consuming to switch the correction amount of the spherical aberration to a target correction amount.
The conventional technique does not consider the deviation in the correction amount of the spherical aberration. The correction amount of the spherical aberration deviates, for example, when the manner of arrangement of the optical disc apparatus is changed, (e.g., when the optical disc apparatus which has been located vertically is located horizontally or vice versa), when a change in the temperature around the optical disc apparatus changes the wavelength of the laser or sensitivity of the control system, or when variance of the spherical aberration correction section occurs during mass production of the spherical aberration correction control device. In such cases, there is a problem in that the spherical aberration cannot be properly corrected.